Conventionally, as a joint in a suspension system and a steering system of an automobile, a ball joint and an arm are often jointed. A conventional example of joint structure of a ball joint and an arm is shown in FIG. 10.
The ball joint 101 shown in FIG. 10 includes: a ball stud 102 having a spherical ball part 104 and a shank part 103 extending from the spherical ball part 104; a bearing 105 made of a synthetic resin, cupping the spherical ball part 104 of the ball stud 102 in such a manner that the spherical ball part 104 is able to turn and rock therein, and having an open part 106 in one end; a housing 107 holding the bearing 105 therein and having a small open part 108 in one end through which the shank part 103 of the ball stud 102 projects and a large open part 109 in the other end; a closing plate 112 fixed to an end portion of the large open part 109 of the housing 107 by caulking; and a dust cover 114 having a dust-cover small open part 115 fitted on the shank part 103 of the ball stud 102 and a dust-cover large open part 116 fitted on the housing 107. Then, the housing 107 of the ball joint 101 is pressed into a hole 120 of an arm 119, so that the arm 119 and the other parts are jointed.
The outer diameter of the housing 107 of the above ball joint 101 is formed to be a little larger than the inner diameter of the hole 120 of the arm 119. Then, when the housing 107 of the ball joint 101 is pressed into the hole 120 of the arm 119, the ball joint 101 and the arm 119 are jointed.
In the joint structure of the ball joint 101 and the arm 119 shown in FIG. 10, the outer diameter of the housing 107 of the ball joint 101 is formed to be a little larger than the inner diameter of the hole 120 of the arm 119. Thus, when the housing 107 is pressed into the hole 120 of the arm 119, the housing 107 receives a pressure from the hole 120 of the arm 119, so that the ball joint 101 is held in the hole 120 of the arm 119.
However, in the joint structure shown in FIG. 10, the press-fitting load given from the hole 120 of the arm 119 to the housing 107 may be also given to the spherical ball part 104 of the ball stud 102 from the housing 107 through the bearing 105. Thus, an operational torque necessary for rocking the ball stud 102 may undesirably become higher than a predetermined value.